24 ECHINOIDEA. 11. 



speciniens of anv Echinothiirid hitherto examined (leaving aside the ver\' doubtful Asthenosotiia bysMx 

 of 3-i'°"'figured in Rev. of Ech., PI. II c.) are those of 3™"" in' diameter described by me (Parti. p. 174). I 

 have stated there that the periproct is, even in the .smallest speciniens, covered by a nmnber of small 

 irregular piates, with no larger between. So a central plate seems never to be found here. Since in 

 this verv \oung stage the anal piates are thiis already present in considerable nnmber and do not show 

 anv trace of five original larger piates covering the whole anal area, I do not think \ve are justified 

 in assnming that these 5 large piates are fonud in a >et earlier stage. I give here a figureofthe anal 

 area of the youngest specimen of Plioniiosoiiia (scarcel\- 3™"') seen b>- nie. (Fig. i.| 



A matter of miich more importance, however, is the statement (p. 91) that in . Pliorinosoviaf 

 hispidum < the bare interambulacral area adjoining the primordial plate is covered with a few minute, 

 elono-ate, irregularh" arranged piates, which correspond to the interradial bnccal piates of Cidaris>. 



The same thing is stated for Kaii/pfosoii/a i)idistiiicfit)ii (p. 112): In this .species we find a few of 



the same irregnlar elongate interambulacral piates which in the 



Cidaridæ are as well and as regularly developed as the ambu- 



■''■'■-'■'- lacral buccal piates . It was hitherto assumed to be one of the 



most important features distinguishing the Cidarida- from all 



the other regular Echinoids that both the ambulacral and inter- 



'"'■■'•''^-:, ambulacral piates continue over the peristome; the Echiiiotliitridæ 



'W^^- were distinguished b\' the ambulacral piates alone continuing 



over the peristome. If these small piates of the peristome found 

 in the two Echinothurids by Professor Agassiz were realh 

 homologous to the interradial buccal piates of the Cidaridæ this 

 ,r fundamental character would have to be gi\en up. Fortunately, 



the figures given bv Agassiz himself afford the proof that 

 Fig. .. .^pical system of a young Phonuo- ^j^^^^ ^^^^ ^^^ ^^^^ homologous with the interradial buccal 



soDia placenta, -^^^^ in diameter. -°/i. ^ ° 



piates of the Cidarids; since the primordial interambulacral 

 plate is persistent, these small piates lying in the buccal membrane inside (adoralh') the primordial 

 plate cannot possibh- have any relation whatever to the interambulacral jDlates and cannot be said to 

 ' correspond to the interradial buccal piates of Cidaris . They correspond to those small, irregular 

 piates found in the peristome of the other regular Echini. 



In treating the Echinothurids in Part I, I had to lea\'e iiiccrtce srdis the species P/ioniiosoiiia.- 

 pduanieiisf and Jiispiduiii. and Professor Agassiz, not recognizing m\- limitation of the genus Plior- 

 mosoma, does not take the trouble to state to which group these species belong. But from the very 

 careful descriptiou and figures of the test combined with my examination of the pedicellariæ of the 

 type specimens in the U. S. National Musetmi, it can be said with certainty that the\- belong to the 

 genus Echinosoiua. It is true that the character of the primår}- actinal spines of paiiavicnsc is unknown, 

 bitt all the other characters are decidedly those of Ecliinosojiia. so that I think we may safely conclude 

 that the spines also are tipped with a hoof and not provided with a fleshy sack. A more detailed de- 

 scriptiou of the pedicellariæ I cannot gi\e on this occasion; it will suffice to say that they agree rather 

 closeh- with those of Ecliinosoma iiraiuis and temie\ in pmKunoisr I have not, however, fomid the 



